From favorite to least favorite, I'd rank the games:
1
3
2
PW
4
MG2: Solid Snake
Ground Zeroes
MG1

I love all of the games, and ranking them is pretty arbitrary for me since they are so close together in my mind. MGS1 was the first I played, and it holds a lot of nostalgic value for that. Recognizing this is with a nostalgic lens, it is pretty close to the perfect game in my mind.

The last three were kind of hard to think about placement. MG1 I just don't remember all that well. MG2 has also not been played in a few years, but I really enjoyed that game. Ground Zeroes just sort of loses points for not being a complete game story-wise. The gameplay is solid and possibly my favorite in the series. My opinion will probably change of that once The Phantom Pain comes out in which I'll probably just consider them one game.

As for which I could live without, it would be the first Metal Gear. It's an important game for the series for obvious reasons, but it's the least fun for me to return to now despite still liking it quite a bit. If we're only counting the Solid series (and withholding judgment on Ground Zeroes for now), I'd say Peace Walker. While I enjoy returning to that one more than MGS4 now, MGS4 is just more important to me story-wise.